1. Field of the Invention
The present invention relates to a variable lens actuator which enables deformation of a lens surface, having an element with a variable optical characteristic, such as a variable-focus lens, an aberration correction lens, etc., and relates to an optical disc drive using the same.
2. Related Art
Recent increase in the amount of electronic information accompanying the penetration of the Internet and the improvement in images for a higher quality has been steadily accelerating higher density of optical discs, which belong to one category of major image recording media. For high density, in general, methods are employed including one that reduces the size of an optical spot of focused light by increasing the number of openings of an objective lens and shortening the wavelength and one that forms multiple recording layers.
Because light from a semiconductor laser is focused on an optical disc through a substrate, spherical aberration occurs, wherein the spherical aberration depends on the wavelength of the laser light, the number of openings of an objective lens, and the thickness of the substrate (the distance between the incident plane of the disc and a recording layer, hereinafter referred to as the substrate thickness). Since this spherical aberration is proportional to the fourth power of the number of openings of the objective lens, the spherical aberration caused by the error variation in the substrate thickness sharply increases with an increase in the number of openings. Further, the difference between the substrate thickness for the recording layer closest to the incident plane of laser beams and that for the recording layer farthest from the incident plane comes to several ten μm (micrometers) as a result of lamination of multiple recording layers, and accordingly, a mechanism is essential that corrects the spherical aberration caused by these to realize normal recording and reproducing operation.
Further, as a material of the objective lens of an optical pickup, glass, plastic, etc. are considered. Plastics are advantageous over glasses in terms of cost, mass-productivity, and the like, and plastic objective lenses are used for many optical pickups in recent years. However, a plastic lens changes much more significantly in the refraction index with the temperature compared with a glass lens, resulting in an increase in the spherical aberration caused by temperature variation. Accordingly, a mechanism for correcting the spherical aberration caused by a change in the temperature is essential particularly for a plastic objective lens.
On the other hand, for an optical pickup, aberrations, such as coma aberration, astigmatism, etc. are caused by errors in the disc tilt of an optical disc with respect to the optical axis of a laser and in the assembly of an optical pickup. Therefore, it is necessary to correct these aberrations.
Spherical aberration is aberration that is rotationally symmetric with respect to the optical axis, and spherical aberration can be made, for example, by moving the lens of an optical pickup in the direction of the optical axis, providing a variable-focus lens on the optical pickup to change the focal length of the variable-focus lens, or the like. On the other hand, coma aberration or astigmatism is aberration that is rotationally asymmetric with respect to the optical axis, and can be made by, for example, using a transparent type liquid crystal element and optimizing a voltage pattern to be applied to the liquid crystal element.
Japanese Patent Application Laid-open no. H13-45067 (Patent Document 1) discloses a technology for an optical pickup, wherein spherical aberration due to error variation in the substrate thickness is corrected by generating spherical aberration with a symbol opposite to the spherical aberration due to the error variation in the substrate thickness, through moving the lens, whose spherical aberration is to be corrected, in the direction of the optical axis with an actuator.
FIG. 15 is a diagram showing an example of an optical system of an optical pickup having a conventional aberration correction mechanism 223.
In order to correct a change in the spherical aberration due to the positional change of a recording layer 211 of an optical disc 212 (the positional change of a recording layer 211a and a recording layer 211b) or the like, conventionally, the divergence angle of a laser beam entering from a laser light source 207 into an objective lens 209 is changed by displacing the position of a lens 221 with respect to the optical axis thereof, using a single axis actuator with a motor 222.
Further, for an optical pickup described in Japanese Patent Application Laid-open No. H12-57616 (Patent Document 2), a technology of correcting spherical aberration by a signal from a detector, using a transparent type liquid crystal element that generates an optical phase difference, is disclosed.
For an optical pickup described in Japanese Patent Application Laid-open No. 2006-155850 (Patent Document 3), a technology of correcting spherical aberration is disclosed wherein the reflection surface of a reflection mirror for reflecting a laser beam to an objective lens is warped into a parabolic shape to change the intensity distribution pattern of laser beams entering an objective lens.
For a liquid crystal aberration correction element having two transparent electrode surfaces, Japanese Patent Application Laid-open No. 2005-122828 (Patent Document 4) discloses a technology, wherein a transparent electrode pattern for correction of coma aberration along the radial direction of an optical disc on one hand and a transparent electrode pattern for correction of coma aberration and astigmatism along the tangential direction of the optical disc on the other hand are provided to generate coma aberration and astigmatism by the liquid crystal aberration correction element, so as to correct coma aberration and astigmatism caused on an optical pickup.